Such apparatuses are known for example from DE 198 40 482 A1 and normally referred to as transmission sensors since light from a light source transmitted by the bank notes to be checked is detected by a sensor and evaluated. A distinction can be made here between light-field and dark-field measurement, depending on whether the illumination is so disposed that the light from the source hits the detector (light-field) or not (dark-field). With dark-field illumination the detector detects only light scattered by the bank note, i.e. changed in its direction.
Known light-field apparatuses are frequently used for recognizing the format of bank notes and/or defects, e.g. tears or holes, by the shadow cast in the bank notes. For this purpose the light background is imperative, on the one hand, to permit e.g. the defects of areas with dark printing to be reliably distinguished. On the other hand, the shadow itself should be as dark as possible. In particular for recognizing small defects (such as pinholes or only slightly open tears) or laser-produced perforations with diameters in the order of magnitude of 0.05 mm, the contrast of the bank notes with the background must be as high as possible. This must also apply to areas with maximum transmission, e.g. areas without printing, or even to transparent areas such as see-through registers on bank notes with a plastic substrate.
Further, in conventional bank note processing systems the transport plane of the bank notes is only imprecisely defined; the bank notes can flutter around a target plane, thereby causing changes in the image distances from illumination and imaging optics. To prevent this flutter from leading to fluctuations in intensity and scale, both optics must be of sufficiently telecentric design, i.e. they may only use light beams that deviate only a few degrees from the parallels to the optical axis.
In known apparatuses for checking bank notes in the light field, the background used is as a rule a line of chip LEDs or backlit surface. This causes the contrast to be adjusted to a lower value than can be obtained with optimal illumination, because the bank note is also illuminated by light beams that, because of their direction relative to the optical axis, can contribute to scattered light but not to the background.